This invention relates generally to medical devices and more particularly to a method and device for delivering ultrasound energy to a treatment location within a human or other mammal.
The use of ultrasound devices for lysing or removing material obstructing blood vessels in humans has been proposed in the art. These devices use ultrasound energy, either alone or with other aspects of a treatment procedure in an attempt to remove material blocking these blood vessels. One such device, an elongated ultrasound transmitting probe, has been used to lyse material obstructing blood vessels of humans or other mammals. The device consists of a cavitation generating tip at the end of an elongated transmission wire. A transducer is used to convert an electrical signal into longitudinal mechanical vibration in the transmission wire. This leads to the generation of a standing wave in the device and longitudinal displacement of the tip to transmit mechanical energy to the obstruction.
It is desirable for such an ultrasound probe to generate a wave with the maximum amplitude with a minimum of applied power. This maximum amplitude will generate the greatest lysing force and energy directed at any material being acted upon in the blood vessel. This will occur when the frequency of the ultrasound applied to the transmission wire of the probe by the transducer approaches the effective resonance frequency of the transmission wire of the probe. However, this effective resonance frequency will vary as the probe is moved within the blood vessel and among different blood vessels. Thus, the transmission wire of the probe may oscillate at less than its maximum amplitude at a given applied power. As a result, the probe will generate less than the maximum amount of ultrasonic energy within the blood vessel. The conditions which may affect the probe normally include bends in the transmission wire and compressions against the wire after the probe is fed through the various blood vessels in the body to the obstruction and moved within the blood vessel during treatment.
Additionally, conventional ultrasound probes do not measure the actual frequency or amplitude of oscillation at the probe tip. For example, space concerns generally preclude the use of features to transmit information regarding the action of the probe tip to a user. Users therefore will generally have no way to know what is actually happening at the probe tip.
One effort at maintaining suitable mechanical power transmitted by the tip is described in U.S. Pat. No. 5,477,509, the contents of which are incorporated herein by reference. This reference describes attempting to control the amplitude of the standing wave in the probe tip by monitoring the current input to the transducer, and varying the power input to the transducer so as to maintain the current input to the transducer at a constant level. Thus, when movement of the probe within the blood vessel decreases the current input to the transducer as a result of a change in the load of the transmission wire on the transducer, the power input to the transducer is increased in an effort to provide a constant power output at the tip of the probe. However, this reference fails to address the cause of the drop in supplied current. Rather the apparatus simply compensates for this decrease by inputting additional power. Thus, more power is required to be input to the transducer for the same output power which results in a decrease in the efficiency of the apparatus.
This prior art reference also describes monitoring the level of current input to the transducer to determine if there is a break in the transmission wire. If a break occurs in the transmission wire, the load of the transmission wire on the transducer will greatly decrease. This results in an extreme decrease in the required power input to achieve the supposed required power output at the tip of the probe. This change signals a problem, and the apparatus is shut down. However, such a system will not detect a problem in the transmission wire, such as a fracture, which might increase the load on the transducer. A fracture might increase the friction between the transmission wire and any other portion of the probe, for example, or any object the probe tip might come into contact with. While this fracture might be dangerous to the user, the required power input would not decrease below a predetermined level, and therefore would not be recognized as an event which would turn off the probe.
The optimal operating frequency of an ultrasonic device varies with the tolerances of the components of the device and the field of operation. In prior art ultrasonic devices, the optimal operating frequency is determined by scanning across the entire operating range of the device and locating the frequency which maximizes a particular operating parameter of the device, e.g. current. A significant drawback associated with the prior art approach of scanning across an entire operating frequency range is that a false optimum frequency may be selected which would result in sub-optimum performance for the device.
Accordingly, it would be beneficial to provide an ultrasound transmission device which can generate a maximum tip oscillation amplitude under a number of adverse conditions, and provide the feedback necessary to maintain maximum amplitude without increasing the power consumption of the apparatus, and which can monitor the system to notify the user of any fracture in the probe wire or other problem affecting the system.
Generally speaking, in accordance with the invention, an ultrasound transmission apparatus in the form of a transmission member connectable to a transducer at its proximal end and having a tip at its distal end is provided. The apparatus includes an improved control system which can control the amplitude of oscillation at the tip of the probe. This control system comprises an electric power source which supplies constant power at a selected frequency to the transducer which converts the electrical energy to mechanical oscillation and generates a standing wave in the transmission member. The control system also includes a frequency measuring and adjusting instrument for continuously measuring the frequency of the mechanical oscillations output from the transducer. This frequency measuring instrument is also capable of varying the frequency of the oscillations of the transmission member and tip by fine tuning the frequency of the oscillations generated by the transducer. Finally, current and voltage monitoring instruments are also included for measuring current and voltage to determine power input to the transducer.
The control system maintains constant power (voltage times current) to the transducer and monitors the current and voltage input to the transducer. The oscillation frequency is varied over a predetermined range in order to maintain a frequency at which current input to the transducer, and thus power, is at a maximum. The resistance along the transmission member during oscillation is proportional to the load on the transducer and therefore electrical resistance at the transducer is proportional to the load on the transducer. Because power is maintained at a constant level, the load on the transducer will be at a minimum at maximum current. The amplitude of the oscillations of the transmission wire will also be at a maximum. Thus, as the frequency of the transducer is constantly adjusted to generate the greatest input current and thus maintain power at its maximum, the apparatus will always optimize the amplitude of the oscillation of the tip thereof at a given power.
This maximum will occur when the transducer vibrates at the effective resonance frequency of the transmission member. As the probe is moved within blood vessels in various parts of the body, the resonance frequency of the probe is slightly altered. By fine tuning the frequency of the oscillation frequency of the transducer, it is possible to oscillate the transmission member at a frequency approaching this new resonance frequency. Therefore, by measuring the input current and voltage to the transducer coupled to the transmission member while fine tuning the oscillation frequency, it is possible to continuously operate the probe at close to the resonance frequency and thus at its maximum power. This will generate the maximum oscillation amplitude at the tip of the transmission member, and insure that the probe is being operated under the predetermined conditions.
Additionally, the invention includes a method for operating an ultrasound transmission device, including the steps of supplying constant electrical power to a transducer of the device and converting this electrical energy to mechanical energy in the form of an oscillating tip thereof. The frequency of oscillation of the transducer is varied over a predetermined range while the current and voltage supplied to the transducer is monitored and the power supplied to the transducer is maintained at a constant maximum level. Then, the value of the frequency which results in the maximum current, and thus power being supplied to the transducer is determined. It is at this frequency, which approaches the resonance frequency of the transmission member, that the resistance to oscillation, and thus impedance of the transducer is at a minimum, and therefore the amplitude of oscillation is at its maximum. By constantly adjusting the frequency of the transducer, and constantly monitoring for any variation in the current input and voltage to the transducer, it is possible to maintain oscillations at the tip of the transmission member at the appropriate amplitude, to insure appropriate ultrasound application to the obstruction.
In an additional embodiment of the invention, an apparatus for monitoring the amplitude, and therefore the ultrasonic energy output by an ultrasound probe, is provided. The apparatus comprises an integrator, which receives a standard voltage input and a feedback signal indicative of the power at the tip of the probe. This voltage signal is then fed into a differential amplifier. This differential amplifier receives input from the integrator, and a feedback error signal, and generates a differential signal which has a compensated value to maintain an accurate frequency signal. This differential signal is then fed to a VCO phase comparator, which compares the frequency of the output signal to the frequency of a reference signal. This reference signal is formed of a first component which defines a predetermined, center frequency of oscillation, and a second component which is a correction based upon the current state of the system, and whether it is necessary to increase or decrease the output frequency. This frequency is then divided by two to yield the adjusted output frequency, because the frequency had previously been maintained at double the required frequency to maintain a higher degree of resolution during measurement and calculation.
This adjusted output frequency signal, which is set to the required frequency, is passed through any number of power amplifiers so that the output signal is always maintained at a constant predetermined power level regardless of the frequency or other factors. This power output is then fed into an additional amplifier which outputs the power to a transducer, which in turn converts this electric power to a mechanical displacement. At the same time, the voltage and current input to the transducer is monitored, and the impedance is determined. These measured values of voltage and current, and the determined value of impedance are fed to a multiplier/filter, which processes the signal to determine the true power output at the transducer, which is also a function of the amplitude of the oscillating tip of the probe. This power determination is then fed back into the integrator where it is processed, and the feedback control loop is completed.
Thus through the use of such an apparatus, it is possible to determine whether the selected oscillation amplitude, and therefore, the selected ultrasonic power is being generated at the tip of an ultrasound probe. It is possible to maximize this power output by fine tuning the frequency of the oscillations within a predetermined range, and monitoring the transducer input current and voltage. The transducer output frequency which generates the greatest current, which takes place at a frequency approaching the resonance frequency of the transmission member in the blood vessel, will also generate the greatest amplitude of oscillation and therefore power output at the probe tip, without adjusting the input power to the transducer. Therefore, the output power from a probe can be safely controlled to within a selected range without expending excess power, and without sacrificing the efficiency of the apparatus.
Accordingly, it is an object of the invention to provide an improved control system for an ultrasound transmission probe.
Another object of the invention is to provide an improved control system and method for an ultrasound probe in which the power efficiency of the probe can be maximized.
Yet another object of the invention is to provide an ultrasound probe which provide a constant output power.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification and the drawings.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.